1. Field of the Invention
The present invention relates to an image-forming apparatus used advantageously for printers or copying machines. In particular, the present invention relates to an electrophotographic image-forming apparatus.
2. Description of the Related Art
Electrophotographic image-forming apparatus have been used in various kinds of printers or copying machines. FIG. 10 of the accompanying drawings shows the basic structure of a conventional image-forming apparatus. The illustrated apparatus includes an image-forming unit 71 to print images on continuous recording paper P, and a paper transfer mechanism 73 to draw out paper P from the paper hopper 72 and feed it to the image-forming unit 71.
The paper transfer mechanism 73 includes scuff rollers 74, back-tension rollers 75, a sub-tractor 76 and a main tractor 77. The scuff rollers 74 exert a forward pulling force on the paper P, while the back-tension rollers 75 exert a backward pulling force (tension) on the paper P. The sub-tractor 76 and the main tractor 77 regulate the transfer speed of the paper P. Each of the tractors 76, 77 includes a pair of tractor belts 77a from which a plurality of tractor pins 77b project. In a paper feeding operation, the pins 77b are brought into engagement with indexing holes of the paper P. FIG. 11 shows a typical indexing hole (reference numeral 78) having a serrated circumference.
The image-forming unit 71 includes a rotatable photosensitive drum 81 upon which an electrostatic latent image is produced. Around the drum 81 are provided a pre-charger 82, a laser emitter 83, a developer 84, a transfer charger 85, an AC discharger 86, a toner cleaner 87 and an LED discharger 88. The functions of these components are as follow. The pre-charger 82 uniformly charges the image-forming surface of the drum 81. The laser emitter 83 irradiates the charge surface with a laser beam to produce an electrostatic latent image in conformity with the original. The latent image is made visible as a toner image by the developer 84. Then, the toner image is transferred onto the paper P by the transfer charger 85. The transferred image is fused to the paper P by application of e.g. heat or pressure from a fixing unit (not shown). The remnant charge on the drum surface is removed by the dischargers 86 and 88, while the remnant toner is removed by the cleaner 87.
In operation, the scuff rollers 74 and the drum 81 are responsible for the forward movement of the paper P, while the back-tension rollers 75 exert a backward force on the paper P so that the paper P is appropriately stretched. To avoid exertion of an unduly strong pulling force on the paper P, the scuff rollers 74 can slip on the paper P. This arrangement is advantageous to avoiding the breakage of the paper P at an indexing hole 78 for example.
While the conventional apparatus is functional, it has the following disadvantage.
In the conventional apparatus, the photosensitive drum 81 can exert a forward pulling force on the paper P while it is held in contact with the paper P. However, at an initial stage of the printing operation where the drum 81 has not come into contact with the paper P yet, the paper P is pulled forward only by the action of the scuff rollers 74. In this situation, the back-tension rollers 75 may pull the paper P backward with a stronger force than the scuff rollers 74 pull the paper P forward. As a result, the paper P may shift backward relative to the main tractor 77, whereby the tractor pins 77b come into contact with the front portion of the indexing holes 78 (see FIG. 11).
When the drum 81 is held in contact with the paper P, on the other hand, the paper P is pulled forward more strongly by the combination of the scuff rollers 74 and the drum 81, whereby the tractor pin 77b is shifted relatively backward in the indexing hole 78. Thus, as shown in FIG. 12, the pin 77b comes into contact with the rear portion of the hole 78.
Disadvantageously, as seen from FIGS. 11 and 12, the tractor pin 77b can shift by a maximum distance (A) through the printing session. This deviation may cause a shear in printing, and therefore should be avoided.
The present invention has been proposed under the circumstances described above. It is, therefore, an object of the present invention to provide an image-forming apparatus which does not suffer the positional deviation of the recording paper relative to the tractor pins.
According to a first aspect of the present invention, there is provided an image-forming apparatus including: a photosensitive member on which an electrostatic latent image is produced; scuff rollers that move recording paper along a transfer path; a tractor disposed upstream of the transfer path from the scuff rollers, the tractor moving the recording paper at a predetermined transfer speed; back-tension rollers disposed upstream from the tractor, the back-tension rollers exerting a tension on the recording paper; and a speed controller that adjusts a transfer speed of the recording paper. The speed controller makes greater the circumferential speed of the back-tension rollers than the transfer speed by the tractor during a period after the recording paper is started to move along the transfer path and before image-printing is begun.
Preferably, the speed controller may make smaller the circumferential speed of the back-tension rollers than the transfer speed by the tractor after the image-printing is begun.
Preferably, the apparatus of the present invention may further include a transfer charger for transferring a toner image onto the recording paper. After a predetermined period of time passes since a beginning of transfer by the transfer charger, the speed controller makes smaller the circumferential speed of the back-tension rollers than the transfer speed by the tractor.
According to a second aspect of the present invention, there is provided an image-forming apparatus including: an image-forming unit that prints an image on recording paper; scuff rollers that move the recording paper along a transfer path; a tractor disposed upstream of the transfer path from the scuff rollers, the tractor moving the recording paper at a predetermined transfer speed; back-tension rollers disposed upstream from the tractor for exerting a tension on the recording paper; and a pressure controller that adjusts a pressing force acting on the recording paper. During a period after the recording paper is started to move along the transfer path and before image-printing is begun, the pressure controller prevents the back-tension rollers from exerting a pressing force on the recording paper.
Preferably, the pressure controller may control the back-tension rollers after the image-printing is begun, so that the back-tension rollers exert a pressing force on the recording paper.
Preferably, the apparatus of the present invention may further include a transfer charger for transferring a toner image onto the recording paper. The pressure controller may cause the back-tension rollers to exert a pressing force on the recording paper after a predetermined period of time passes since the beginning of image transfer performed by the transfer charger.
Preferably, the apparatus of the present invention may further include a mechanism for changing the position of the back-tension rollers. The pressure controller controls the position-changing mechanism to cause the back-tension rollers to selectively exert a pressing force on the recording paper.
Preferably, the back-tension rollers may include a drive roller and a follower roller which faces the drive roller and is moved by the position-changing mechanism.
According to a third aspect of the present invention, there is provided an image-forming apparatus including: an image-forming unit provided with a photosensitive member on which an electrostatic latent image is produced; scuff rollers that move recording paper along a transfer path; a tractor disposed upstream of the transfer path from the scuff rollers, the tractor moving the recording paper along the transfer path at a predetermined transfer speed; back-tension rollers disposed upstream from the tractor for exerting a tension on the recording paper; and a pull controller that adjusts a pulling force acting on the recording paper. The pull controller makes greater a pulling force of the scuff rollers than a pulling force of the back-tension rollers during a period after the recording paper is started to move along the transfer path and before image-printing is begun.
Preferably, in the above apparatus, the pull controller may make greater the sum of the pulling force of the scuff rollers and pulling force of the photosensitive member than the pulling force of the back-tension rollers after the image-printing is begun.
Preferably, the pull controller may make greater the pulling force of the scuff rollers during a period after the recording paper is started to move along the transfer path and before image-printing is begun than after the image-forming is begun.
Preferably, the above apparatus may further include a transfer charger for transferring a toner image onto the recording paper. The pull controller may make greater the sum of the pulling force of the scuff rollers and the pulling force of the photosensitive member than the pulling force of the back-tension rollers after a predetermined period of time passes since a beginning of transfer by the transfer charger.
Preferably, the above apparatus may further include a pressure adjusting mechanism for the scuff rollers. In this case, the pull controller may control the pressure adjusting mechanism, thereby changing the pressing force of the scuff rollers acting on the recording paper.
Preferably, the scuff rollers may include a drive roller and a follower roller which faces the drive roller and is associated with the pressure adjusting mechanism.
Preferably, the above-mentioned predetermined period of time may be determined depending on the forward pulling force of the photosensitive member acting on the recording paper.
Other features and advantages of the present invention will become apparent from the detailed description given below with reference to the accompanying drawings.